Dc/dc converter

ABSTRACT

A pulse modulator generates a pulse modulation signal S PM  having a duty ratio which is adjusted according to a feedback voltage Vfb that corresponds to the output voltage VOUT of a DC/DC converter. A second oscillator generates a second cyclic signal which is asserted with each of a predetermined second period. A light load detection circuit generates a light load detection signal which is asserted when the feedback voltage Vfb becomes lower than a first threshold voltage. A driving circuit drives a switching transistor according to the pulse modulation signal S PM , Furthermore, the driving circuit suspends the driving of the switching transistor during a period until the second cyclic signal is next asserted after the light load detection signal is asserted.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a DC/DC converter.

2. Description of the Related Art

Various kinds of consumer electronics devices such as TVs,refrigerators, etc., are each configured to operate receiving commercialAC electric power from an external circuit. Also, electronic devicessuch as laptop computers, cellular phone terminals, and PDAs (PersonalDigital Assistants) are each configured to operate using commercial ACelectric power, and/or to be capable of charging a built-in batteryusing such commercial AC electric power. Such consumer electronicsdevices and electronic devices (which will collectively be referred toas “electronic devices” hereafter) each include a built-in power supplyapparatus (inverter) configured to perform AC/DC conversion ofcommercial AC voltage. Alternatively, such an inverter is configured asa built-in component included within an external power supply adapter(AC adapter) for such an electronic device.

Related techniques are disclosed in Japanese Patent ApplicationLaid-Open No. H09-098571, and Japanese Patent Application Laid-Open No.H02-211055, for example.

FIG. 1 is a block diagram showing an inverter investigated by thepresent inventor. An inverter 1 r mainly includes a fuse 2, an inputcapacitor Ci, a filter 4, a diode rectifier circuit 6, a smoothingcapacitor Cs, and a DC/DC converter 10 r.

The commercial AC voltage V_(AC) is input to the filter 4 via the fuse 2and the input capacitor Ci. The filter 4 is configured to remove noiseincluded in the commercial AC voltage V_(AC). The diode rectifiercircuit 6 is configured as a diode bridge circuit configured to performfull-wave rectification of the commercial AC voltage V_(AC). The outputvoltage of the diode rectifier circuit 6 is smoothed by the smoothingcapacitor Cs, thereby generating a converted DC voltage VH.

The insulated DC/DC converter 10 r is configured to receive the DCvoltage VH via an input terminal P1, to step down the DC voltage VH thusreceived, and to supply an output voltage VOUT stabilized to the targetvalue to a load (not shown) connected to an output terminal P2.

The DC/DC converter 10 r includes a control circuit 100 r, an outputcircuit 200, and a feedback circuit 210. The output circuit 200 includesa transformer T1, a first diode D1, a first output capacitor CM, aswitching transistor M1, and a detection resistor RS. The output circuit200 has a typical topology, and accordingly, detailed descriptionthereof will be omitted.

An output terminal (OUT terminal) of the control circuit 100 r isconnected to the gate of the switching transistor M1 via a resistor Rg.The control circuit 100 r is configured to perform switching of theswitching transistor M1 so as to step down the input voltage VH, therebygenerating the output voltage VOUT. Furthermore, by adjusting the dutyratio of the switching operation of the switching transistor M1, thecontrol circuit 100 r is configured to stabilize the output voltage VOUTto a target value, and to control a coil current ILp that flows througha primary winding W1 of the transformer T1.

The detection resistor RS is arranged in series with the primary windingW1 of the transformer T1 and the switching transistor M1. A voltage drop(detection voltage) V_(CS), which is proportional to the current ILpthat flows through the primary winding W1 and the switching transistorM1, develops across the detection resistor RS. The detection voltageV_(CS) is input to a current detection terminal (CS terminal) of thecontrol circuit 100 r. The control circuit 100 r is configured tocontrol, based on the detection voltage V_(CS), the current ILp thatflows through the primary winding W1.

The feedback circuit 210 is configured to generate a feedback voltageVfb that corresponds to the output voltage VOUT, and to supply thefeedback voltage Vfb thus generated to a feedback terminal (FB terminal)of the control circuit 100 r. The feedback circuit 210 includes a shuntregulator 212 and a photocoupler 214. The shunt regulator 212 isconfigured as an error amplifier, and is configured to generate afeedback signal S11 having a level adjusted such that the differencebetween the output voltage VOUT and a predetermined target valueapproaches zero, and to supply the feedback signal S11 thus generated toa light-emitting diode of the photocoupler 214. A phototransistor (orotherwise a photodiode) of the photocoupler 214 is configured to converta light signal S12 received from the light-emitting diode into thefeedback voltage Vfb that corresponds to the feedback signal S11.

On the primary winding side, the transformer T1 includes an auxiliarywinding W3, in addition to the primary winding W1. The auxiliary windingW3, a second diode D2, and a second output capacitor C_(VCC) form asecond DC/DC converter. At the second output capacitor C_(VCC), a DCvoltage V_(cc) develops according to the switching of the switchingtransistor M1. The DC voltage V_(CC) is supplied to a power supplyterminal VCC (VCC terminal) of the control circuit 100 r. A startresistor Rstart is arranged between the VCC terminal and the inputterminal P1. In the starting operation, the capacitor C_(VSS) is chargedvia the start resistor Rstart, thereby supplying the power supplyvoltage V_(CC) to the control circuit 100 r.

The control circuit 100 r is configured as a so-called peak current modepulse modulator. Specifically, the control circuit 100 r includes anedge blanking circuit 102, a pulse modulator 110, a driving circuit 130,and a light load detection circuit 140.

Immediately after the switching transistor M1 is turned on, thedetection voltage V_(CS) temporarily jumps. In order to preventundesired turning-off of the switching transistor M1 due to such a jumpin the detection voltage V_(CS), the edge blanking circuit 102 isconfigured to mask the detection voltage V_(CS) during a mask periodimmediately after the switching transistor M1 is turned on.

A capacitor Cfb is connected to an FB terminal as an external component.Furthermore, the FE terminal is pulled up via a resistor R11. Thefeedback voltage Vfb is divided by means of the resistors R12 and R13.

The pulse modulator 110 is configured to generate a pulse signal S_(PM)having a duty ratio which is adjusted according to the feedback voltageVfb. The pulse modulator 110 is configured to control the timing atwhich the switching transistor M1 is turned off, according to thedetection voltage V_(CS) which is proportional to the coil current ILpthat flows through the switching transistor M1. Known examples of such apulse modulator 110 include an average current mode modulator, a peakcurrent mode modulator, and a fixed off-time mode pulse modulator. Adriver 104 is configured to perform switching of the switchingtransistor M1 according to the pulse signal S_(PM).

The pulse modulator 110 shown in FIG. 1 is configured as a peak currentmode modulator, and includes an error comparator 112, an oscillator 114,and a logic unit 116. The error comparator 112 is configured to comparea divided feedback voltage Vfb′ with a detection voltage V_(CS)′. Theerror comparator 112 is configured to generate an off signal S_(OFF)which is asserted when the detection voltage V_(CS)′ reaches thefeedback voltage Vfb′.

The oscillator 114 is configured to generate an ON signal S_(ON) whichis asserted with a predetermined cycle. The logic unit 116 is configuredas an SR flip-flop. The logic unit 116 is arranged such that the ONsignal S_(ON) is input to its set terminal, and an OFF signal S_(OFF) isinput to its reset terminal. An output signal (which will be referred toas a “pulse modulation signal”) S_(PM) of the logic unit 116 transits tothe on level (high level) that corresponds to the on state of theswitching transistor M1 every time the ON signal S_(ON) is asserted.Furthermore, the pulse modulation signal S_(PM) transits to the offlevel (low level) that corresponds to the off state of the switchingtransistor M1 every time the OFF signal S_(OFF) is asserted.

The driving circuit 130 is configured to perform switching of theswitching transistor M1 according to the pulse signal S_(PM). Thedriving circuit 130 includes a pre-driver 132 and a driver 134.

The light load detection circuit 140 is configured to detect a lightload state in which the DC/DC converter 10 r outputs a reduced outputcurrent. The light load detection circuit 140 is configured to generatea light load detection signal S140 which is asserted (set to low level)when the DC/DC converter 10 r enters the light load state. The lightload detection circuit 140 includes a burst comparator 142 and aninverter 144.

When, in the light load state, the output current of the DC/DC converter10 r falls, the output voltage VOUT rises, and the feedback voltage Vfbdrops. Thus, the burst comparator 142 is configured to compare thefeedback voltage Vfb′ with a predetermined threshold voltage Vth.Furthermore, the burst comparator 142 is configured such that, when Vfb′becomes smaller than Vth, it judges that the DC/DC converter 10 r hasentered the light load state. The inverter 144 is configured to invertthe logic level of the light load detection signal S140.

The pre-driver 132 is configured to suspend the switching of theswitching transistor M1 during a period in which the light loaddetection signal S140 is asserted.

In the heavy load state, the DC/DC converter 10 r shown in FIG. 1operates at a switching frequency that is the same as the frequency ofthe ON signal S_(ON) generated by the oscillator 114. Typically, thefrequency of the ON signal S_(ON) is set to a frequency that issufficiently higher than the audible band. For example, the frequency ofthe ON signal S_(ON) is set to a frequency on the order of 50 to 100kHz.

In the light load state, the switching transistor M1 performs a burstswitching operation according to the light load detection signal S140.In this state, the switching rate of the switching transistor M1 isreduced. This reduces the charge/discharge current that flows to/fromthe gate capacitance of the switching transistor M1, thereby providingimproved efficiency.

With such an arrangement, the period in which the light load detectionsignal S140 is asserted in the light load state, i.e., a period in whichthe switching operation is suspended, becomes longer as the supply ofelectric power to the load becomes smaller. Thus, in the light loadstate, the effective switching frequency of the switching transistor M1becomes lower as the period of suspension of the switching operationbecomes longer, and at length it enters the audible band, which is equalto or below 20 kHz, leading to acoustic noise. In particular, suchacoustic noise ranging between 4 kH and 20 KHz is also referred to as“mosquito noise”, which is very unpleasant. Thus, there is a demand forreducing such acoustic noise.

Also, the length of the switching noise suspension period, i.e., theswitching frequency in the light load state, depends on the capacitanceof the capacitor CM and the input voltage VH, in addition to theelectric power supplied to the load. The capacitance of the capacitor CMand the input voltage VH differ according to the platforms on which thecontrol circuit 100 r is operated. Thus, it becomes necessary to providenoise countermeasures according to the platform.

SUMMARY OF THE INVENTION

The present invention has been made in order to solve such a problem.Accordingly, it is an exemplary purpose of an embodiment of the presentinvention to provide a DC/DC converter which is capable of reducing suchunpleasant acoustic noise.

An embodiment of the present invention relates to a control circuit fora DC/DC converter. The DC/DC converter comprises a switching transistorand a detection resistor arranged on a path of the switching transistor.The control circuit comprises: an error comparator configured to comparea feedback voltage that corresponds to an output voltage of the DC/DCconverter with a detection voltage across the detection resistor, and togenerate an OFF signal which is asserted according to a comparisonresult; a first oscillator configured to generate a first cyclic signalwhich is asserted with each of a predetermined first period; a logicunit configured to generate a pulse modulation signal having a levelthat is switched according to the OFF signal and the first cyclicsignal; a second oscillator configured to generate a second cyclicsignal which is asserted with each of a second period that is longerthan the first period; a light load detection circuit configured togenerate a light load detection signal which is asserted when thefeedback voltage becomes lower than a predetermined first thresholdvoltage; and a driving circuit configured to drive the switchingtransistor according to the pulse modulation signal, and to suspend thedriving of the switching transistor during a period until the secondcyclic signal is next asserted after the light load detection signal isasserted.

Another embodiment of the present invention also relates to a controlcircuit for a DC/DC converter. The DC/DC converter comprises atransformer, a switching transistor arranged on a current path of aprimary winding of the transformer, and a detection resistor. Thetransformer comprises an auxiliary winding provided on its primarywinding side. The control circuit comprises: an error comparatorconfigured to compare a feedback voltage that corresponds to an outputvoltage of the DC/DC converter with a detection voltage across thedetection resistor, and to generate an OFF signal which is assertedaccording to a comparison result; an ON signal generating unitconfigured to generate an ON signal which is asserted when a voltage atone end of the auxiliary winding becomes lower than a predeterminedsecond threshold voltage; a logic unit configured to generate a pulsemodulation signal having a level that is switched according to the OFFsignal and the ON signal; a second oscillator configured to generate asecond cyclic signal which is asserted with each of a predeterminedsecond period; a light load detection circuit configured to generate alight load detection signal which is asserted when the feedback voltagebecomes lower than a predetermined first threshold voltage; and adriving circuit configured to drive the switching transistor accordingto the pulse modulation signal, and to suspend the driving of theswitching transistor during a period until the second cyclic signal isnext asserted after the light load detection signal is asserted.

Yet another embodiment of the present invention also relates to acontrol circuit for a DC/DC converter. The control circuit comprises: apulse modulator configured to generate a pulse modulation signal havinga duty ratio that is adjusted based on a feedback voltage thatcorresponds to an output voltage of the DC/DC converter; a secondoscillator configured to generate a second cyclic signal which isasserted with each predetermined second period; a light load detectioncircuit configured to generate a light load detection signal which isasserted when the feedback voltage becomes lower than a predeterminedfirst threshold voltage; and a driving circuit configured to drive theswitching transistor according to the pulse modulation signal, and tosuspend the driving of the switching transistor during a period untilthe second cyclic signal is next asserted after the light load detectionsignal is asserted.

With such embodiments, even if the light load state continues and thelight load detection signal is continuously asserted for a long periodof time, the switching transistor is turned on once with each secondperiod. Thus, the switching frequency at which the switching frequencyoperates in the light load state can be set according to the secondperiod. Thus, such an arrangement suppresses the occurrence ofundesired, unpleasant acoustic noise.

Also, the control circuit may be configured such that it ismonolithically integrated on a single semiconductor substrate.

Examples of such a “monolithically integrated” arrangement include: anarrangement in which all the circuit components are formed on asemiconductor substrate; and an arrangement in which principal circuitcomponents are monolithically integrated. Also, a part of circuitcomponents such as resistors and capacitors may be arranged in the formof components external to such a semiconductor substrate in order toadjust the circuit constants. By integrating such a control circuit inthe form of a single IC (Integrated Circuit), such an arrangementprovides a reduced circuit area while maintaining improved uniformity ofthe circuit element characteristics.

Another embodiment of the present invention relates to a DC/DCconverter. The DC/DC converter comprises: a transformer comprising aprimary winding and a secondary winding; a switching transistorconnected to the primary winding of the transformer; a first diodehaving its anode connected to the secondary winding; a first outputcapacitor having its one electrode grounded, and its other electrodeconnected to the cathode of the first diode; a feedback circuitconfigured to generate a feedback voltage that corresponds to the outputvoltage across the first output capacitor; and the control circuitaccording to any one of the aforementioned embodiments configured toreceive the feedback voltage and to perform switching of the switchingtransistor.

Also, the feedback circuit may comprise: a shunt regulator configured togenerate a feedback signal having a level that is adjusted such that thedifference between a voltage obtained by dividing the output voltage anda predetermined target value approaches zero; and a photocouplercomprising a primary-side light-emitting element configured to becontrolled according to the feedback signal. Also, a signal generated bya secondary-side element of the photocoupler may be supplied as thefeedback voltage to the control circuit.

Also, the transformer may further comprise an auxiliary winding on itsprimary winding side. Also, the DC/DC converter further may comprise: asecond diode having its anode connected to the auxiliary winding; and asecond output capacitor having its one electrode grounded, and its otherelectrode connected to a cathode of the second diode. Also, a DC voltageacross the second output capacitor may be supplied to a power supplyterminal of the control circuit.

Yet another embodiment of the present invention relates to a powersupply apparatus. The power supply apparatus comprises: a filterconfigured to perform filtering of a commercial AC voltage; a dioderectifier circuit configured to perform full-wave rectification of anoutput voltage of the filter; a smoothing capacitor configured to smoothan output voltage of the diode rectifier circuit, so as to generate a DCinput voltage; and the DC/DC converter according to any one of theaforementioned embodiments, configured to step down the DC inputvoltage, and to supply the DC input voltage thus stepped down to a load.

Yet another embodiment of the present invention relates to an electronicdevice. The electronic device comprises: a load; a filter configured toperform filtering of a commercial AC voltage; a diode rectifier circuitconfigured to perform full-wave rectification of an output voltage ofthe filter; a smoothing capacitor configured to smooth an output voltageof the diode rectifier circuit, so as to generate a DC input voltage;and the DC/DC converter according to any one of the aforementionedembodiments, configured to step down the DC input voltage, and to supplythe DC input voltage thus stepped down to the load.

Yet another embodiment of the present invention relates to a powersupply adapter. The power supply adapter comprises: a filter configuredto perform filtering of a commercial AC voltage; a diode rectifiercircuit configured to perform full-wave rectification of an outputvoltage of the filter; a smoothing capacitor configured to smooth anoutput voltage of the diode rectifier circuit, so as to generate a DCinput voltage; and the DC/DC converter according to any one of theaforementioned embodiments, configured to step down the DC input voltageso as to generate the DC output voltage.

It is to be noted that any arbitrary combination or rearrangement of theabove-described structural components and so forth is effective as andencompassed by the present embodiments.

Moreover, this summary of the invention does not necessarily describeall necessary features so that the invention may also be asub-combination of these described features.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying drawings which are meant to be exemplary,not limiting, and wherein like elements are numbered alike in severalFigures, in which:

FIG. 1 is a circuit diagram showing an inverter investigated by thepresent inventor;

FIG. 2 is a circuit diagram showing a configuration of a DC/DC converterincluding a control circuit according to an embodiment;

FIG. 3 is a waveform diagram showing the operation of the DC/DCconverter shown in FIG. 2 in the light load state;

FIGS. 4A and 4B are diagrams respectively showing the relation betweenthe output voltage, the feedback voltage, and the switching frequency inthe DC/DC converters shown in FIGS. 1 and 2;

FIG. 5 is a diagram showing an AC adapter including an inverter;

FIGS. 6A and 6B are diagrams each showing an electronic device includingan inverter;

FIG. 7 is a circuit diagram showing a configuration of a control circuitaccording to a first modification; and

FIGS. 8A and 8B are circuit diagrams showing the configurations of thepulse modulators according to the second and third modifications,respectively.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described based on preferred embodiments whichdo not intend to limit the scope of the present invention but exemplifythe invention. All of the features and the combinations thereofdescribed in the embodiment are not necessarily essential to theinvention.

In the present specification, the state represented by the phrase “themember A is connected to the member B” includes a state in which themember A is indirectly connected to the member B via another member thatdoes not affect the electric connection therebetween, in addition to astate in which the member A is physically and directly connected to themember B.

Similarly, the state represented by the phrase “the member C is providedbetween the member A and the member B” includes a state in which themember A is indirectly connected to the member C, or the member B isindirectly connected to the member C via another member that does notaffect the electric connection therebetween, in addition to a state inwhich the member A is directly connected to the member C, or the memberB is directly connected to the member C.

FIG. 2 is a circuit diagram showing a configuration of a DC/DC converter10 including a control circuit according to an embodiment.

The DC/DC converter 10 includes a control circuit 100, an output circuit200, and a feedback circuit 210. The output circuit 200 and the feedbackcircuit 210 have the same configurations as those shown in FIG. 1.

Description will be made below regarding a configuration of the controlcircuit 100.

The control circuit 100 is configured as a function IC monolithicallyintegrated on a single semiconductor substrate. It should be noted thatdescription of the same components as those shown in FIG. 1 will beomitted.

The pulse modulator 110 is configured as a peak current mode modulator,and includes an error comparator 112, an oscillator 114, and a logicunit 116. The error comparator 112 is configured to compare a feedbackvoltage Vfb′ with a detection voltage V_(CS)′, and to generate an OFFsignal S_(OFF) which is asserted (set to high level) When Vfb′ becomessmaller than V_(CS)′.

The oscillator 114 is configured to generate a first cyclic signal (ONsignal) S_(ON) which is asserted with each of a predetermined firstperiod. The logic unit 116 is configured to generate a pulse modulationsignal S_(PM) having a level that is switched according to the OFFsignal S_(OFF) and the first cyclic signal S_(ON). The logic unit 116may be configured as an SR flip-flop. Also the logic unit 116 may beconfigured using other kinds of logic elements.

In the same way as with the light load detection circuit 140 shown inFIG. 1, a light load detection circuit 140 according to the presentembodiment is configured to compare the feedback voltage Vfb′ with apredetermined first threshold voltage Vth1 so as to detect a light loadstate. The light load detection circuit 140 is configured to assert (setto low level) a light load detection signal S140 when a light load stateis detected.

The control circuit 100 shown in FIG. 2 further includes a secondoscillator 150 and a logic gate 152, in addition to the configuration ofthe control circuit 100 r shown in FIG. 1.

The second oscillator 150 is configured to generate a second cyclicsignal OSC2 which is asserted (set to high level) with each of apredetermined second period that is longer than the first period. Forexample, the first cyclic signal S_(ON) is configured to have afrequency Fsw1 of 25 KHz, and accordingly, the first period is 40 μs. Onthe other hand, the second cyclic signal OSC2 is configured to have afrequency Fsw1 of 4 KHz, and accordingly, the second period is 250 μs.

A driving circuit 130 is configured to drive a switching transistor M1according to the pulse modulation signal S_(PM). The driving circuit 130is configured to suspend the driving of the switching transistor M1during a period until the second cyclic signal OSC2 is next assertedafter the light load detection signal S140 is asserted.

The logic gate 152 is configured to receive the second cyclic signalOSC2 and the light load detection signal S140. Furthermore, the logicgate 152 is configured to generate a burst control signal S152 whichtransits to a first level (high level) when the second cyclic signalOSC2 is asserted, and which transits to a second level (low level) whenthe light load detection signal S140 is asserted. The driving circuit130 is configured to drive the switching transistor M1 during a periodin which the burst control signal S152 is set to the first level (highlevel), and to suspend the driving of the switching transistor M1 duringa period in which the burst control signal S152 is set to the secondlevel (low level).

The logic gate 152 is configured as a D flip-flop, for example. The Dflip-flop is arranged such that a high level voltage (e.g., power supplyvoltage Vcc) is input to its input terminal (D), the second cyclicsignal OSC2 is input to its clock terminal, and the light load detectionsignal S140 is input to its reset terminal (inverted logical signal).The driving circuit 130 is configured to suspend the driving of theswitching transistor M1 during a period in which the output signal S152of the logic gate 152 is set to low level, and to perform switching ofthe switching transistor M1 according to the pulse modulation signalS_(PM) during a period in which the output signal S152 of the logic gate152 is set to high level.

Also, the logic gate 152 may be configured as another kind of logicgate, e.g., it may be configured as an RS flip-flop instead of such a Dflip-flop, which is readily understood by those skilled in this art.

The above is the configuration of the control circuit 100. Next,description will be made regarding the operation thereof. FIG. 3 is awaveform diagram showing the operation of the DC/DC converter 10 shownin FIG. 2 in the light load state.

When the ON signal S_(ON) is asserted at the time point to, the pulsemodulation signal S_(PM) transits to the on level (high level), whichsets the output signal S_(OUT) to high level, thereby turning on theswitching transistor M1. When the switching transistor M1 is turned on,the current ILp that flows through the primary winding W1 increases overtime. Furthermore, the detection voltage V_(CS) rises such that it isproportional to the current ILp. When the detection voltage V_(CS)′reaches the feedback voltage Vfb′ at the time point t1, the pulsemodulation signal S_(PM) transits to the off level, which turns off theswitching transistor M1.

When the switching transistor M1 is turned off, the first outputcapacitor CM is charged by the current ILs that flows through thesecondary winding W2, which raises the output voltage VOUT. The feedbacksignal Vfb′ falls according to the rise of the output voltage VOUT.Subsequently, the light load detection signal S140 is asserted at thetime point t2. When the light load detection signal S140 is asserted,the burst control signal S152, which is the output of the logic gate152, is switched to low level. When the burst control signal S152 isswitched to low level, the switching of the switching transistor M1 issuspended.

After the switching of the switching transistor M1 is suspended, theoutput voltage VOUT starts to fall, which raises the feedback voltageVfb′. When the feedback voltage Vfb′ becomes higher than the thresholdvoltage Vth1 at the time point t3, the light load detection signal S140is negated (set to low level).

Subsequently, when the first cyclic signal S_(ON) is asserted at thetime point t4, the pulse modulation signal S_(PM) is switched to the onlevel, which turns the switching transistor M1 on again. Furthermore,when the second cyclic signal OSC2 is asserted at the time point 4, theburst control signal S152 is asserted, which instructs the drivingcircuit 130 to restore the switching of the switching transistor M1.

The control circuit 100 is configured to repeatedly perform theaforementioned operation.

The advantages of the control circuit 100 will be clearly understood incomparison with the control circuit 100 r shown in FIG. 1.

FIGS. 4A and 4B are diagrams showing the relation between the outputvoltage, the feedback voltage, and the switching frequency, in a case ofemploying the DC/DC converters shown in FIGS. 1 and 2, respectively.

With the control circuit 100 shown in FIG. 1, both the pulse width andthe frequency of the light load detection signal S140 change accordingto a change in the output voltage. Thus, with the control circuit 100 rshown in FIG. 1, as shown in FIG. 4A, the switching frequency changes ina range that includes the audible band according to a reduction in theoutput power in the light load state when the output power is small.

In contrast, with the control circuit 100 shown in FIG. 2, the burstcontrol signal S152 is asserted once with each second period. That is tosay, the frequency (burst frequency) of the burst control signal S152 isfixed to the frequency of the second cyclic signal OSC2. That is to say,only the pulse width of the burst control signal S152 changes accordingto the output power. As a result, the switching frequency is maintainedat the frequency Fsw2, which is the frequency of the second cyclicsignal OSC2, even if the output power falls.

Thus, by setting the frequency Fsw2 of the second cyclic signal OSC2 toa frequency that is lower than the frequency band that can be recognizedby the user as unpleasant audible noise which should be avoided, e.g.,by setting the frequency Fsw2 of the second cyclic signal OSC2 to 4 kHz,such an arrangement is capable of reducing such unpleasant noise.Furthermore, even if such acoustic noise occurs, the frequency of suchacoustic noise is a constant value that can be predicted beforehand.Thus, such an arrangement facilitates the implementation ofcountermeasures with respect to such acoustic noise, which is anotheradvantage.

Description will be made regarding the usage of the DC/DC converter 10.An inverter 1 including the DC/DC converter 10 is suitably employed inan AC adapter and a power supply block of an electronic device.

FIG. 5 is a diagram showing an AC adapter 800 including the inverter 1.The AC adapter 800 includes a plug 802, a casing 804, and a connector806. The plug 802 is configured to receive commercial AC voltage V_(AC)from an unshown electrical outlet. The inverter 1 is mounted within thecasing 804. The DC output voltage VOUT generated by the inverter 1 issupplied to an electronic device 810 via the connector 806. Examples ofsuch an electronic device 810 include laptop PCs, digital still cameras,digital video cameras, cellular phone terminals, portable audio players,etc.

FIGS. 6A and 6B are diagrams each showing an electronic device 900including the inverter 1. The electronic device 900 shown in FIGS. 6Aand 6B is configured as a display apparatus. However, the electronicdevice 900 is not particularly restricted in kind, as long as itincludes a power supply apparatus as an internal component. Examples ofsuch an electronic device 900 include audio devices, refrigerators,washing machines, vacuum cleaners, etc.

A plug 902 is configured to receive commercial AC voltage V_(AC) from anunshown electrical outlet. The inverter 1 is mounted within the casing804. The DC output voltage VOUT generated by the inverter 1 is suppliedto loads mounted within the same casing 904, examples of which include amicrocomputer, DSP (Digital Signal Processor), power supply circuit,illumination device, analog circuit, digital circuit, etc.

Description has been made regarding the present invention with referenceto the embodiment. The above-described embodiment has been described forexemplary purposes only, and is by no means intended to be interpretedrestrictively. Rather, it can be readily conceived by those skilled inthis art that various modifications may be made by making variouscombinations of the aforementioned components or processes, which arealso encompassed in the technical scope of the present invention.Description will be made below regarding such modifications.

[First Modification]

Description has been made in the embodiment regarding the controlcircuit 100 using the pulse width modulation method. However, thepresent invention is not restricted to such an arrangement. Also, thepresent invention can be applied to a control circuit using the pulsefrequency modulation method.

FIG. 7 is a circuit diagram showing a configuration of a control circuit100 a according to a first modification. The control circuit 100 aincludes a ZT terminal. The voltage Va at one end of an auxiliarywinding W3 is divided by resistors Rzt1 and Rzt2. The voltage Vzt thusdivided is input to the ZT terminal.

A pulse modulator 110 a includes an ON signal generating unit 118instead of the oscillator 114 shown in FIG. 2. The ON signal generatingunit 118 is configured to generate an ON signal S_(ON) which is assertedwhen the voltage Va at the aforementioned one end of the auxiliarywinding W3 becomes lower than a predetermined second threshold voltageVth2.

During a period in which the switching transistor M1 is on, energy isstored in the transformer T1. On the other hand, during a period inwhich the switching transistor M1 is off, energy is discharged from thetransformer T1. Immediately after the switching transistor M1 is turnedoff, the current ILs flows through the secondary winding W2 during acertain period. During this period, the voltage Va is maintained at acertain voltage level. When the energy stored in the transformer T1approaches zero, the current ILs also approaches zero. In this stage,the voltage Va fluctuates due to quasi-resonance. When the voltage Vafalls to the vicinity of zero due to such fluctuation, the ON signalgenerating unit 118 is configured to judge that the energy stored in thetransformer T1 has become zero, and to assert the ON signal S_(ON) inorder to turn the switching transistor M1 on again.

The configuration and the operation of the ON signal generating unit 118are not restricted in particular, and may be configured using knowntechniques. For example, when the ON signal generating unit 118 detectsthat the voltage Va at the aforementioned one end of the auxiliarywinding W3 has become lower than the second threshold voltage Vth2 apredetermined number of times, the ON signal generating unit 118 may beconfigured to assert the ON signal S_(ON). In this case, the ON signalgenerating unit 118 may be configured using a comparator configured tocompare the voltage Vzt with a threshold voltage, and a counterconfigured to count the transitions of the output of the comparator.

With another modification, the ON signal generating unit 118 may beconfigured to assert the ON signal S_(ON) when it detects that thevoltage Va at the aforementioned one end of the auxiliary winding W3becomes lower than the second threshold voltage Vth2 after a certain setmask period elapses after the switching transistor M1 is turned off.

The above is the configuration of the control circuit 100 a. The controlcircuit 100 a thus configured provides the same advantages as thoseprovided by the control circuit 100 shown in FIG. 2.

[Second Modification]

Description has been made in the embodiment regarding the peak currentmode pulse modulator 110. However, the present invention is notrestricted to such an arrangement. FIG. 8A is a circuit diagram showinga configuration of a pulse modulator 110 b according to a secondmodification. The pulse modulator 110 b shown in FIG. 8A is configuredas a so-called average current mode modulator. An error amplifier 120 isconfigured to generate an error voltage Verr obtained by amplifying andaveraging the difference between the feedback voltage Vfb′ and thedetection voltage V_(CS)′. An oscillator 122 is configured to generate acyclic signal OSC1 having a triangle waveform or otherwise a sawtoothwaveform with a predetermined frequency Fsw1. A PWM (pulse widthmodulation) comparator 124 is configured to compare the error voltageVerr with the cyclic signal OSC1, and to generate a pulse modulationsignal S_(PM) that is pulse-width modulated according to the comparisonresult. With such an average current mode modulator, such an arrangementprovides the same advantages as those provided by the embodiment.

[Third Modification]

FIG. 8B is a circuit diagram showing a configuration of a pulsemodulator 110 c according to a third modification. The pulse modulator110 c shown in FIG. 8B is configured as a so-called fixed off-time modepulse modulator.

The error comparator 112 is configured to compare the feedback voltageVfb′ with the detection voltage V_(CS)′, and to generate an OFF signalS_(OFF) which is asserted according to the comparison result. An offtime fixation circuit 126 is configured to generate a pulse modulationsignal S_(PM) which is switched to the off level (e.g., high level)which corresponds to the off state of the switching transistor M1 whenthe off signal S_(OFF) is asserted, and which is switched to the onlevel (e.g., low level) which corresponds to the on state of theswitching transistor M1 after a predetermined off time T_(OFF) elapsesafter the pulse modulation signal S_(PM) is switched to the off level.With such a fixed off-time pulse mode modulator, such an arrangementprovides the same advantages as those provided by the embodiment.

[Fourth Modification]

Description has been made in the embodiment and in the first throughthird modifications regarding an insulated DC/DC converter. Also, thepresent invention is applicable to a non-insulated DC/DC converter. Inthis case, such an arrangement includes an inductor instead of thetransformer T1 included in the output circuit 200.

[Fifth Modification]

Description has been made in the embodiment regarding a step-down DC/DCconverter. Also, the present invention is applicable to a step-up DC/DCconverter. With such an arrangement, the topology of the output circuit200 may preferably be modified.

[Sixth Modification]

Description has been made in the embodiment regarding an arrangement inwhich the shunt regulator (error amplifier) 212 is provided on thesecondary side of the transformer T1. Also, such an error amplifier maybe provided on the primary side of the transformer T1. Also, such anerror amplifier may be built into the control circuit 100.

Description has been made in the embodiment regarding an arrangementemploying a positive logic (active high) system in which the assertstate of each signal is associated with the high level, and the negatestate of each signal is associated with the low level. Also, anarrangement may be made employing a negative logic system. Also, such anarrangement may employ a combination of the positive logic system andthe negative logic system.

Description has been made regarding the present invention with referenceto the embodiments using specific terms. However, the above-describedembodiments show only the mechanisms and applications of the presentinvention for exemplary purposes only, and are by no means intended tobe interpreted restrictively. Rather, various modifications and variouschanges in the layout can be made without departing from the spirit andscope of the present invention defined in appended claims.

What is claimed is:
 1. A control circuit for a DC/DC convertercomprising a switching transistor and a detection resistor arranged on apath of the switching transistor, the control circuit comprising: anerror comparator configured to compare a feedback voltage thatcorresponds to an output voltage of the DC/DC converter with a detectionvoltage across the detection resistor, and to generate an OFF signalwhich is asserted according to a comparison result; a first oscillatorconfigured to generate a first cyclic signal which is asserted with eachof a predetermined first period; a logic unit configured to generate apulse modulation signal having a level that is switched according to theOFF signal and the first cyclic signal; a second oscillator configuredto generate a second cyclic signal which is asserted with each of asecond period that is longer than the first period; a light loaddetection circuit configured to generate a light load detection signalwhich is asserted when the feedback voltage becomes lower than apredetermined first threshold voltage; and a driving circuit configuredto drive the switching transistor according to the pulse modulationsignal, and to suspend the driving of the switching transistor during aperiod until the second cyclic signal is next asserted after the lightload detection signal is asserted.
 2. The control circuit according toclaim 1, further comprising a logic gate configured to receive thesecond cyclic signal and the light load detection signal, and togenerate a burst control signal which is switched to a first level whenthe second cyclic signal is asserted, and which is switched to a secondlevel when the light load detection signal is asserted, wherein thedriving circuit is configured to drive the switching transistor during aperiod in which the burst control signal is set to the first level, andto suspend the driving of the switching transistor during a period inwhich the burst control signal is set to the second level.
 3. Thecontrol circuit according to claim 2, wherein the logic gate comprises aD flip-flop arranged such that a high level voltage is input to itsinput terminal, the second cyclic signal is input to its clock terminal,and the light load detection signal is input to its reset terminal.
 4. Acontrol circuit for a DC/DC converter comprising a transformer, aswitching transistor arranged on a current path of a primary winding ofthe transformer, and a detection resistor, wherein the transformercomprises an auxiliary winding provided on its primary winding side, andwherein the control circuit comprises: an error comparator configured tocompare a feedback voltage that corresponds to an output voltage of theDC/DC converter with a detection voltage across the detection resistor,and to generate an OFF signal which is asserted according to acomparison result; an ON signal generating unit configured to generatean ON signal which is asserted when a voltage at one end of theauxiliary winding becomes lower than a predetermined second thresholdvoltage; a logic unit configured to generate a pulse modulation signalhaving a level that is switched according to the OFF signal and the ONsignal; a second oscillator configured to generate a second cyclicsignal which is asserted with each of a predetermined second period; alight load detection circuit configured to generate a light loaddetection signal which is asserted when the feedback voltage becomeslower than a predetermined first threshold voltage; and a drivingcircuit configured to drive the switching transistor according to thepulse modulation signal, and to suspend the driving of the switchingtransistor during a period until the second cyclic signal is nextasserted after the light load detection signal is asserted.
 5. Thecontrol circuit according to claim 4, further comprising a logic gateconfigured to receive the second cyclic signal and the light loaddetection signal, and to generate a burst control signal which isswitched to a first level when the second cyclic signal is asserted, andwhich is switched to a second level when the light load detection signalis asserted, and wherein the driving circuit is configured to drive theswitching transistor during a period in which the burst control signalis set to the first level, and to suspend the driving of the switchingtransistor during a period in which the burst control signal is set tothe second level.
 6. The control circuit according to claim 5, furthercomprising a D flip-flop arranged such that a high level voltage isinput to its input terminal, the second cyclic signal is input to itsclock terminal, and the light load detection signal is input to itsreset terminal.
 7. The control circuit according to claim 4, wherein theON signal generating unit is configured to assert the ON signal when itdetects that a voltage at one end of the auxiliary winding has becomelower than the second threshold voltage a predetermined number of times.8. The control circuit according to claim 4, wherein the ON signalgenerating unit is configured to assert the ON signal when it detectsthat a voltage at one end of the auxiliary winding has become lower thanthe second threshold voltage after a set mask period elapses after theswitching transistor is turned off.
 9. A control circuit for a DC/DCconverter comprising a switching transistor and a detection resistorarranged on a path of the switching transistor, the control circuitcomprising: a pulse modulator configured to generate a pulse modulationsignal having a duty ratio that is adjusted based on a feedback voltagethat corresponds to an output voltage of the DC/DC converter; a secondoscillator configured to generate a second cyclic signal which isasserted with each predetermined second period; a light load detectioncircuit configured to generate a light load detection signal which isasserted when the feedback voltage becomes lower than a predeterminedfirst threshold voltage; and a driving circuit configured to drive theswitching transistor according to the pulse modulation signal, and tosuspend the driving of the switching transistor during a period untilthe second cyclic signal is next asserted after the light load detectionsignal is asserted.
 10. The control circuit according to claim 9,wherein the pulse modulator comprises a peak current mode pulsemodulator.
 11. The control circuit according to claim 9, wherein thepulse modulator comprises: an error comparator configured to compare afeedback voltage that corresponds to an output voltage of the DC/DCconverter with a detection voltage across the detection resistor, and togenerate an OFF signal which is asserted according to a comparisonresult; a first oscillator configured to generate a first cyclic signalwhich is asserted with each predetermined first period; and a logic unitconfigured to generate a pulse modulation signal having a level that isswitched according to the OFF signal and the first cyclic signal. 12.The control circuit according to claim 9, wherein the DC/DC converter isconfigured as an insulated DC/DC converter comprising a transformer thatcomprises an auxiliary winding on its primary winding side, and whereinthe pulse modulator comprises: an error comparator configured to comparea feedback voltage that corresponds to an output voltage of the DC/DCconverter with a detection voltage across the detection resistor, and togenerate an OFF signal which is asserted according to a comparisonresult; an ON signal generating unit configured to generate an ON signalwhich is asserted when a voltage at one end of the auxiliary windingbecomes lower than a predetermined threshold voltage; and a logic unitconfigured to generate a pulse modulation signal having a level which isswitched according to the OFF signal and the ON signal.
 13. The controlcircuit according to claim 12, wherein the ON signal generating unit isconfigured to assert the ON signal when it detects that a voltage at oneend of the auxiliary winding has become lower than the second thresholdvoltage a predetermined number of times.
 14. The control circuitaccording to claim 12, wherein the ON signal generating unit isconfigured to assert the ON signal when it detects that a voltage at oneend of the auxiliary winding has become lower than the second thresholdvoltage after a set mask period elapses after the switching transistoris turned off.
 15. The control circuit according to claim 9, wherein thepulse modulator comprises an average current mode pulse modulator. 16.The control circuit according to claim 9, wherein the pulse modulatorcomprises: an error amplifier configured to generate an error voltageobtained by amplifying and averaging a difference between a feedbackvoltage that corresponds to an output voltage of the DC/DC converter anda detection voltage across the detection resistor; and a pulse widthmodulation comparator configured to compare the error voltage with acyclic signal having a predetermined cycle and having a trianglewaveform or otherwise a sawtooth waveform, and to generate a pulsemodulation signal according to a comparison result.
 17. The controlcircuit according to claim 9, wherein the pulse modulator comprises afixed off-time mode pulse modulator.
 18. The control circuit accordingto claim 9, wherein the pulse modulator comprises: an error comparatorconfigured to compare a feedback voltage that corresponds to an outputvoltage of the DC/DC converter with a detection voltage across thedetection resistor, and to generate an OFF signal which is assertedaccording to a comparison result; and an OFF time fixation circuitconfigured to generate a pulse modulation signal which is switched to anoff level that corresponds to an off state of the switching transistorwhen the OFF signal is asserted, and which is switched to an on levelthat corresponds to an on state of the switching transistor after apredetermined off time elapses after the pulse modulation signal isswitched to off level.
 19. The control circuit according to claim 1,configured such that it is monolithically integrated on a singlesemiconductor substrate.
 20. A DC/DC converter comprising: a transformercomprising a primary winding and a secondary winding; a switchingtransistor connected to the primary winding of the transformer; adetection resistor arranged on a path of the switching transistor; afirst diode having its anode connected to the secondary winding; a firstoutput capacitor having its one electrode thereof grounded, and itsother electrode connected to a cathode of the first diode; a feedbackcircuit configured to generate a feedback voltage that corresponds to anoutput voltage across the first output capacitor; and a control circuitconfigured to receive the feedback voltage, and to perform switching ofthe switching transistor, and wherein the control circuit comprises: anerror comparator configured to compare a feedback voltage thatcorresponds to an output voltage of the DC/DC converter with a detectionvoltage across the detection resistor, and to generate an OFF signalwhich is asserted according to a comparison result; a first oscillatorconfigured to generate a first cyclic signal which is asserted with eachof a predetermined first period; a logic unit configured to generate apulse modulation signal having a level which is switched according tothe OFF signal and the first cyclic signal; a second oscillatorconfigured to generate a second cyclic signal which is asserted witheach of a second period that is longer than the first period; a lightload detection circuit configured to generate a light load detectionsignal which is asserted when the feedback voltage becomes lower than apredetermined first threshold voltage; and a driving circuit configuredto drive the switching transistor according to the pulse modulationsignal, and to suspend the driving of the switching transistor during aperiod until the second cyclic signal is next asserted after the lightload detection signal is asserted.
 21. The DC/DC converter according toclaim 20, wherein the feedback circuit comprises: a shunt regulatorconfigured to generate a feedback signal having a level that is adjustedsuch that the difference between a voltage obtained by dividing theoutput voltage and a predetermined target value approaches zero; and aphotocoupler comprising a primary-side light-emitting element configuredto be controlled according to the feedback signal, and wherein a signalgenerated by a secondary-side element of the photocoupler is supplied asthe feedback voltage to the control circuit.
 22. The DC/DC converteraccording to claim 20, wherein the transformer further comprises anauxiliary winding on its primary winding side, and wherein the DC/DCconverter further comprises: a second diode having its anode connectedto the auxiliary winding; and a second output capacitor having its oneelectrode grounded, and its other electrode connected to a cathode ofthe second diode, and wherein a DC voltage across the second outputcapacitor is supplied to a power supply terminal of the control circuit.23. A power supply apparatus comprising: a filter configured to performfiltering of a commercial AC voltage; a diode rectifier circuitconfigured to perform full-wave rectification of an output voltage ofthe filter; a smoothing capacitor configured to smooth an output voltageof the diode rectifier circuit, so as to generate a DC input voltage;and the DC/DC converter according to claim 20, configured to step downthe DC input voltage, and to supply the DC input voltage thus steppeddown to a load.
 24. An electronic device comprising: a load; a filterconfigured to perform filtering of a commercial AC voltage; a dioderectifier circuit configured to perform full-wave rectification of anoutput voltage of the filter; a smoothing capacitor configured to smoothan output voltage of the diode rectifier circuit, so as to generate a DCinput voltage; and the DC/DC converter according to claim 20, configuredto step down the DC input voltage, and to supply the DC input voltagethus stepped down to the load.
 25. A power supply adapter comprising: afilter configured to perform filtering of a commercial AC voltage; adiode rectifier circuit configured to perform full-wave rectification ofan output voltage of the filter; a smoothing capacitor configured tosmooth an output voltage of the diode rectifier circuit, so as togenerate a DC input voltage; and the DC/DC converter according to claim20, configured to step down the DC input voltage so as to generate theDC output voltage.